There are many electronic devices and apparatus, especially in consumer electronics, that use a battery (cell) as their power supply. The voltage that can be provided by the cell does not necessarily match the voltage used in electronic devices. Thus, a need exists to convert the voltage, i.e., increase the voltage for supply to electronic circuitry. Step-up circuits, also called up-converters, have traditionally been used wherein two step-up circuits are connected in parallel in order to accommodate particularly low input voltages. FIG. 3 illustrates a step-up circuit comprising two parallel stages.
In FIG. 5, coil 1 and transistor 2 are connected in series between power supply line 3 and ground 4, while an anode of diode 5 is connected to a node formed by coil 1 and transistor 2. The output of oscillator circuit 6 is connected to a gate of transistor 2, turning transistor 2 on and off. Step-up circuits 7 and 8 configured in this way are connected in two parallel stages, with cathodes of diodes 5 and 9 being coupled together, from which increased output voltage is extracted. The extracted output is supplied to smoothing circuit 10, where a smoothed DC current is taken from output 11. The output of smoothing circuit 10 is provided to level sense circuit 13 and pulse width control circuit 14, respectively.
A pulse provided from oscillator circuit 6 switches transistor 2 from on to off, a voltage higher than that on power supply line 3 is developed at node of coil 1 and diode 5, and decreases gradually. Then, upon turn-on of transistor 2, the voltage becomes approximately equal to ground voltage. Therefore, at the anode of diode 5 is developed a voltage fluctuation corresponding to the pulse provided by oscillator circuit 6, and when the fluctuation is rectified by diode 5, a voltage higher than that on power supply line 3 is obtained. When the smoothed voltage provided by diode 5 becomes higher than a predetermined voltage level, transistor 2 is turned off, so that a control pulse continues to be supplied from oscillator circuit 6 to transistor 12 via pulse width control circuit 14.
The second-stage step-up circuit 8 performs a similar operation to that of step-up circuit 7, to provide an increased voltage from diode 9. Level sense circuit 13, also called a shut-down circuit, provides a low-level voltage to gate 15 and stops a supply of pulses from oscillator circuit 6 to transistor 2 when the output voltage from output 11 reaches a predetermined level. Upon termination of supply of pulses, a control pulse from pulse width control circuit 14 is supplied to the gate of transistor 12, and step-up circuit 8 starts its operation. Pulse width control circuit 14 controls, in response to an input voltage, i.e., the output voltage of output 11, the pulse width of the control pulse. That is, when the output voltage becomes lower than a target voltage level, it controls the pulse width so that the turn-on period for transistor 12 is extended, whereas when the voltage becomes higher than the target level, it controls the pulse width so that the turn-off period for transistor 12 is extended. Even if the load fluctuates in that way, the output voltage is controlled so that it is maintained constant.
As described above, connecting step-up circuits in two parallel stages and switching them from the first stage to the second permits a relatively low voltage to be converted to a target voltage level. However, since a rush current to the step-up circuits is developed upon switching to the second-stage step-up circuit, a large current will temporarily tend to draw momentarily from the cell. As a result, the output voltage of the cell is decreased due to an internal resistance of the cell, thus reducing the output voltage of the voltage converter. When the output voltage of the cell is too low, it is impossible to smoothly switch to the second-stage step-up circuit, resulting in, for example, unstable oscillation of the oscillator circuit, and, in the worst-case scenario, failure of the voltage converter to operate.